None of the hospitalized people suffered life-threatening injuries, officials said.

The explosion happened in the back of the building, possibly as a result of equipment testing that was going on. The entire plant was evacuated, and everyone was directed to the opposite side of the street.

Emergency crews were so worried about fumes and chemicals after the explosion, they wore protective hazmat suits.

Michael Swanson, of Buffalo Grove, said, “It sounded kind of like gunshots, kind of – like, a mixture of gunshots and, like, a train, kind of. It was, like a big loud boom.”

The blast could have been worse. The destructive force of the explosion was contained somewhat, because the machine that blew up was housed in a caged off section of the plant.

Still, there was widespread damage

“There is a lot of destruction and devastation in there, from the equipment that exploded,” Galinski said

Arlington Heights Fire Chief Glenn Ericksen said, “It also damaged the roof. It threw rocks off the roof. So it was a very violent explosion.”

The first police officers on the scene rushed into the smoke-filled building to guide employees to safety, and to oversee evacuation of the structure.

Eriksen said inside, there was a lot of twisted metal.

Former WBBM Newsradio technician Al Rosen said around 8:30 a.m., people in the neighborhood heard “a very, very loud explosion.” Within 10 minutes, ambulances were pulling up, and employees were being led from the building.

“They were in various states of confusion, and they were placed on the sidewalk, and the ambulance came and picked them up,” Rosen said.

Rosen said he did not notice any fumes. But the an employee of the Nissan dealership next door to the Arens plant said there was a strong odor that she could not describe.

Potassium hydroxide — a strong base that is highly reactive toward acids and highly corrosive — was being used at the time and that’s one of the reasons why emergency workers were in hazmat suits.

“At this point in time, we don’t know what role the chemical had in the accident,” Ericksen said.

Potassium hydroxide is useful in the manufacturing of soap, bleach, batteries and food additives – among other products – but can cause an emission of flammable hydrogen gas if it reacts with certain metals.

The U.S. Occupational Safety and Health Administration opened an investigation and had inspectors at the site.